The mammalian sirtuin family includes seven nicotinamide adenine dinucleotide (NAD)-dependant deacetylases (SirT1-7), which are implicated in the extension of life span (Michan et al., Biochem J, 404(1): 1-13 (2007); Taylor et al., Chem Med Chem., 2(10): 1419-31 (2007)). SirT1 has been comprehensively studied, and multiple targets such as histone, p53, FOXO, PGC1-α have been identified. By deacetylating and interacting with cellular proteins, SirT1 regulates diverse cellular functions including DNA transcription, stress resistance, apoptosis, glucose homeostasis, insulin secretion and fatty acid metabolism. In contrast, the information about SirT6 is very limited. It is known that SirT6 knock-out mice demonstrated the most striking phenotype among the sirtuins (Lombard et al., J. Intern. Med., 263(2): 128-41 (2008)). SirT6-deficient mice are small and at 2-3 weeks of age develop abnormalities. They have a profound decrease in lymphocyte numbers, low glucose and IGF-1 levels in serum, and low subcutaneous fat (Mostoslaysky et al., Cell, 124(2): 315-29 (2006)). At about 4 weeks, SirT6-deficient mice eventually died. It was demonstrated that SirT6 knock-out mice and cells were very sensitive to DNA damage. SirT6 was shown to be required for the stable association with telomeres of the protein WRN, a DNA metabolic factor that is mutated in the human progeria Werner Syndrome (Michishita et al., Nature, 452(7186): 492-496 (2008)); the DNA DSB repair factor, DNA-dependent protein kinase (DNA-PK) at DSBs (Michishita et al., Cell Cycle, 8(16): 2664-2666 (2009)); and the RELA subunit of NF-κB, a transcription factor involved in apoptosis, cell senescence, inflammation and immunity. These accumulative discoveries reveal the importance of SirT6 in the telemere function, DNA damage repair, genomic stability and stress resistance.
As a nucleus protein, SirT6 was found to be associated with a chromatin-enriched cellular fraction (Mostoslaysky et al., Cell, 124(2): 315-329 (2006)). Another study demonstrated that SirT6 bound to nucleosomes (Michishita et al., Cell Cycle, 8(16), 2664-2666 (2009)) and possessed deacetylation activity on histone H3 lysine 9 (Michishita et al., Nature, 452(7186): 492-496 (2008)). This deacetylation activity is very specific, as SirT6 did not deacetylate the acetyl groups at H2A, H2B, H4 and other positions of H3 (Michishita et al., Nature, 452(7186): 492-496 (2008)). Except for telomeric chromatin, SirT6 deacetylates H3K9 at NF-κB target gene promoters, attenuating NF-κB signaling. These established enzymatic properties of SirT6 opened the window to decipher the cellular functions of SirT6. However, the kinetic and biochemistry properties about this enzyme are still lacking, especially from the standing point of enzymology.
The modulators of sirtuins have attracted a lot of attention and interest in recent years. For example, nicotinamide, a product in the sirtuin reaction, was found to inhibit the activity of yeast Sir2 and other sirtuins to some extent (Sauve et al., Annu. Rev. Biochem., 75: 435-465 (2006)). Resveratrol, a natural chemical found in grape seed, was shown to activate SirT1. These modulators play important roles in the cellular metabolism (Sauve et al., supra). Recently, SRT1720, a more potent activator of SirT1, was discovered after screening a large compound library and it has been shown to be useful in the improvement of whole-body glucose homeostasis and insulin sensitivity in adipose tissue, skeletal muscle and liver (Milne et al., Nature, 450(7170): 712-716 (2007)). To date, no modulators have been reported for SirT6.